Near-unity circular dichroism and superchiral enhancement in quasi-BIC dielectric metasurface.

The intrinsically weak chirality of natural materials severely limits the probing of chiral light-matter interactions, which motivates the development of nanophotonic platforms for enhancing and controlling optical chirality. This work proposes and investigates an asymmetric double-slot disk (ADSD) dielectric metasurface to achieve strong intrinsic optical chirality and super-chiral near-field enhancement. The symmetric DSD is designed to support a true bound state in the continuum (BIC), which is confirmed through eigenmode simulations. Breaking the diagonal inversion symmetry transforms the ideal BIC into a quasi-BIC that exhibits near-unity circular dichroism (CD) with a quality factor (Q-factor) of ∼1000. Moreover, near-field simulation reveals that the quasi-BIC strongly confines chiral electromagnetic fields, showing an enhancement of the optical chirality density up to a factor of ∼40. Furthermore, introducing a weak chiral medium into the near field transfers the enhanced optical chirality to the far field, providing a substantial CD enhancement of ∼3000 compared to the structure without the metasurface. The near-unity chirality and strong enhancement of optical chirality density in the proposed metasurface enables high sensitivity chiral sensing and spectroscopy of weak chiral media.